I can answer part of your question: Casting numbers often have little to do with service part numbers. If a part is offered as a casting only, then the service part number will be the same as the casting number. If, however, the casting is sold with some part added, like a bushing pressed in, two or more bits welded together, or if it comes packaged with something else, then the service part number will be different. Another tidbit: When a service part number has a single letter prefix, it is a single thing, whether a casting, forging, steel stamping, rubber o-ring, etc. Any assembly of two or more components will have a two-letter prefix. If the part number has 5 or more digits, the final letter is dropped.

The service part number you mention, RE46990, starts life as a casting (probably numbered with just an "R" prefix), then it is machined and I guess maybe a pivot pin welded/pressed in. At that point it gets a two-digit service part number. Sometimes, the casting number will show up in the parts catalog description as 'marked R12345'. If more than one 'marked' numbers appears in the description, that indicates that there were various updates throughout production.

If you want the latest, the greatest, the last and the best, then order RE46990 from your friendly neighborhood Deere dealer and be assured. But as long as the used one fits your machine and changes the gears correctly, I wouldn't worry about the casting numbers matching.

Sorry, all the above doesn't really help you know how the right part may be marked. For that, you might need to talk to an old mechanic. Like I used to tell them when they grumbled about (occasionally) getting the wrong part, "Hey, I'm the parts guy. I got you the parts. It's up to you to make 'em fit..." ;>)